1. The Field of the Invention
The present invention relates to producing hydrogen peroxide by means of the so-called anthraquinone process. More specifically, the invention relates to an essential unit process of the said method, i.e. hydrogenation. The invention also relates to a hydrogenation reactor.
2. The Background Art
In the prior art it is known that hydrogen peroxide is produced by the so-called anthraquinone process. This process is based on alternate hydrogenation and oxidation of anthraquinone derivatives, generally alkyl anthraquinones. Alkyl anthraquinones are present in the process as dissolved in a solvent formed of several organic substances. This solution, which is called the working solution, circulates continuously through the most important steps of the process. In the hydrogenation step, the alkyl anthraquinones are catalytically hydrogenated into alkyl anthrahydroquinones: ##STR1## In the next step, oxidation, alkyl anthrahydroquinones react with oxygen, whereupon they are returned to their original state, i.e. to alkyl anthraquinones. At the same time, there is created hydrogen peroxide. Oxidation is followed by extraction, where the hydrogen peroxide dissolved in the working solution is extracted therefrom with water. An aqueous solution of hydrogen peroxide is thus obtained. The extracted working solution is dried of excess water and recycled to the beginning of the process, i.e. to hydrogenation. The aqueous solution of hydrogen peroxide is purified and concentrated. More detailed information of the anthraquinone process is given for instance in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A13, pages 447-456.
The hydrogenation step in the hydrogen peroxide process has been realized with many different types of reactors. Part of these are so-called suspension reactors, where catalyst particles move in the working solution, part are fixed bed reactors, where the catalyst remains in place in stationary structures.
When comparing various hydrogenation reactors, one of the most important criteria is the product yield per reactor volume or per catalyst mass unit. Another important feature is good selectivity, in other words a desired ratio between the conversions of the main reaction and the sidereactions. The quantity of wasted hydrogen must also be small, which means that a good yield must be achieved with a minimal hydrogen excess. Other essential differences are found in the auxiliary equipment required for hydrogenation, such as the catalyst filtration equipment, as well as in the hydrogenation-maintaining operations, such as catalyst regeneration. Consequently, there are several factors relevant for the comparison, and certainly none of the used reactors is best in all respects. A relatively comprehensive view in the comparison is naturally achieved by studying the overall costs of hydrogenation.
The yield and selectivity of the reactor are mainly dependent on the hydrogenation pressure and temperature, the concentrations of the reacting substances, the activity of the catalyst, the mixing conditions and the retention time of the reacting mixture in the reactor. The properties of the catalyst have a decisive effect on the rate of the reaction itself. Another factor affecting the rate of hydrogenation, apart from the reaction rate itself, is mass transfer, particularly the transfer of hydrogen from gas to liquid and further to the surface of the catalyst. Thus it is important to provide advantageous conditions for mass transfer.
As for yield and selectivity, there exists an optimal pressure and temperature for hydrogenation. In order to carry out the hydrogenation process as near to these conditions as possible, it is advantageous that the reactor pressure and temperature fluctuate within a range as narrow as possible.
The employed suspension catalyst has been porous palladium (so-called palladium black) or raney nickel or, in some cases, palladium on a carrier (active carbon, aluminium oxide). When using a suspension catalyst, the hydrogenation reactor can be for instance of the stirred tank type. There is also known a suspension reactor operated on the air lift principle (GB patent 718,307). There are further known tubular reactors, where mixing is achieved either by turbulence caused by a high flow velocity (U.S. Pat. No. 4,428,923), by adjustments in the tube diameter (U.S. Pat. No. 3,423,176) or by static mixers (NL patent application 8702882).
Suspension reactors do, however, have several drawbacks in comparison with fixed bed reactors. Firstly, when using a suspension catalyst, there is needed effective filtration after hydrogenation, because none of the catalyst must be allowed to enter the next step of the process, i.e. oxidation. In this case filtration is fairly expensive and technically problematic, requiring complicated circulation arrangements. Filtration is made particularly difficult by the fact that the catalyst particles are extremely small.
When a suspension reactor is used, a large part, even the major part of the expensive catalyst may be in some other area than the reactor itself, in other words in the filtration equipment, in the circulation tank or in the connected pipework. Part of the catalyst may remain stuck on the stationary surfaces of these fixtures for remarkable lengths of time. Thus only part of the catalyst is found in the reaction space, where it is useful.
A third drawback of the suspension reactor is the greater susceptibility of the catalyst to mechanical wearing. This may be one of the reasons for the known fact that the catalyst in a fixed bed reactor generally maintains its activity longer than the suspension catalyst.
These three drawbacks do not appear, if the catalyst is attached to stationary support structures, in which case the reactor is a fixed bed reactor. A traditional fixed bed reactor contains layers formed of particles, generally with a diameter of 0.2-10 mm. Sometimes the particles are round, sometimes grains or pellets of indeterminate shape. The carrier in these particles is some porous substance with a large specific surface, such as aluminium oxide, activated carbon or silica gel. In the carrier, there is absorbed some precious metal to serve as the catalyst, in this case usually palladium. In hydrogenation, the working solution and the hydrogen flow concurrently or countercurrently through the catalyst layer. This type of fixed bed reactor is always referred to below when speaking of a "traditional fixed bed reactor".
The traditional fixed bed reactor described above suffers from several drawbacks weakening the efficiency of the hydrogenation. First of all, the transfer of hydrogen from gas to liquid, and in the liquid further to the surface of the catalyst, is not very fast in an apparatus of this type. Thus a patent publication describing the said reactor type (U.S. Pat. No. 1,227,047, page 2, column 2, lines 74-81) states that the dissolution of hydrogen takes place at a lower rate than the reaction itself in the catalyst. This is a weakness of the traditional fixed bed type reactors as compared with suspension reactors, for example. In order to minimize the amount of the expensive catalyst, the hydrogen should dissolve so quickly that it would not noticeably restrict the overall rate of the hydrogenation process. This is the case with the most advanced suspension reactors, for instance with the reactor type described in the NL patent application 8702882.
Another drawback of the traditional fixed bed reactor type is that the flow is easily channelled in the catalyst layer. This means that a high flow velocity prevails in some places, whereas in other places the flow may be practically nonexistent. Channelling may also lead to a situation where the gas finds its own routes through the catalyst layer, and consequently does not flow evenly everywhere. Channelling naturally reduces the overall rate of the hydrogenation process.
Although the activity of the catalyst generally lasts better in the fixed bed reactor than in the suspension reactor, the activity finally wears out irrespective of the reactor type. Then the catalyst must be removed and regenerated. In most fixed bed reactors this is an arduous operation, which can be considered as a drawback in comparison with suspension reactors. In traditional fixed bed reactors, the removal of the catalyst mass and reinstallation of a new catalyst easily takes a few days. Usually the catalyst bound to a carrier must be sent back to the producer, and a replacement is bought from the same. Consequently, in the case of a fixed bed reactor, the regeneration of the catalyst often means exchanging it. When using a suspension reactor, the replacement of the catalyst can be carried out gradually, without stopping the reactor. Moreover, if there is used a catalyst without a carrier (for instance palladium black), the regeneration thereof can be carried out fairly easily without resorting to the catalyst producer enterprises.
In order to help dissolve the hydrogen, the U.S. Pat. No. 3,565,581 suggests a fixed bed reactor with alternate catalyst layers and inert carrier layers. This arrangement, however, increases the reactor volume, and hence also the quantity of the expensive working solution circulating in the process.
Another suggestion to help dissolve hydrogen is disclosed in the U.S. Pat. No. 2,837,411, where the working solution is saturated with hydrogen in a separate tank prior to the reactor. The use of this technique also leads to an increase in the amount of the working solution. Moreover the usefulness of the predissolution is fairly limited, because a multiple quantity of hydrogen is consumed in the reactor in comparison with the amount that is made to dissolve to the working solution at a time. For the same reason, a very limited advantage is gained from the invention introduced in the U.S. Pat. No. 4,428,922, where hydrogen is premixed to the working solution prior to the reactor by means of a static mixer.
As a conclusion of the comparison of traditional fixed bed reactors with suspension reactors, it is maintained that the former have at least the following advantages:
Firstly, in a fixed bed reactor the required filtration of the catalyst is generally a lesser and cheaper operation. In suspension reactors, filtration can be extremely difficult, complicated and expensive.
Secondly, in a fixed bed reactor all of the catalyst is inside the reactor, where hydrogen also is found, and the catalyst is in use. When using a suspension reactor, catalyst is found mixed in the liquid in other parts, too, for instance in the circulation tank, filtration equipment and connected pipework. In the latter case there is thus needed an excess of the expensive catalyst, in addition to the quantity which is in effective use.
Thirdly, in a fixed bed reactor the catalyst generally retains its activity longer than in a suspension reactor.
On the other hand, at least two drawbacks can be associated with the traditional fixed bed reactor. First of all, the hydrogen transfer from gas to liquid and further to the catalyst is slower than in the best suspension reactors, which are described in the NL patent application 8702882 and in the U.S. Pat. No. 4,428,923. Secondly, the regeneration of the catalyst is more cumbersome than for instance when using palladium black in a suspension.
As an arrangement different from the traditional fixed bed reactor there is suggested a reactor with a honeycomb structure (U.S. Pat. No. 2,837,411, FI patent 89,787 and FI patent 88,701). Here the catalyst bed is constructed by providing the reactor with one or several honeycomb-type catalyst elements, so that these elements form parallel channels, wherethrough the working solution is circulated several times, advantageously concurrently with hydrogen. The catalyst is attached to the walls of the channels. A better hydrogenation result is achieved with an application where the honeycomb structure contains, in addition to the channels parallel to the main flow, channels perpendicular thereto, and where also mixing is provided within the reactor, for instance by means of static mixers (FI patent 88,701).
The advantages of the honeycomb structure are probably based on hydrogen transfer. There hydrogen is made to proceed from gas through the working solution to the catalyst faster than in a traditional fixed bed reactor. Channelling is not a danger either, if the liquid is originally distributed evenly to the various channels of the honeycomb structure.
The yields of honeycomb structured reactors and traditional fixed bed reactors can be compared on the basis of information given in published patents. In a honeycomb structured reactor (European patent application 0,384,905), the reported yield is 133 kg H.sub.2 O.sub.2 / (m.sup.3 h), where the yield is thus calculated per volume unit of the reactor structure. A reported yield for a traditional fixed bed reactor (U.S. Pat. No. 3,009,782, FIG. 1) is about 1 mol H.sub.2 O.sub.2 /h per each liter of the catalyst bed, which amounts to 34 kg/ (h m.sup.3) only. Partly this difference can naturally be explained by a different activity of the catalyst itself.
The honeycomb structured reactor also has its drawbacks. Presumably the most essential of these is connected to the regeneration of the catalyst. The honeycomb structured catalyst elements, whether ceramic or metallic in frame, require a complicated production technique. In addition to this, they are expensive. The removal of passivated catalyst from the walls of the channels of the structures is probably not possible without breaking up the whole catalyst honeycomb. In any case, the regeneration of the catalyst is an arduous and very costly operation, when this type of reactor application is used.
Another drawback here is the distribution of the liquid into the honeycomb catalyst reactor. The U.S. Pat. No. 4,552,748 specifies that the channels are advantageously 1-2 mm in diameter. The liquid is attempted to be distributed from the top evenly into these tiny channels. According to the FI patent 89,787, the employed distributor can be a sieve bottom, i.e. a perforated plate. The liquid flows through the perforations in the sieve bottoms as trickles and hits the honeycomb structure located therebelow. In order to make the liquid distribute evenly into the adjacent channels with a diameter of 1-2 mm located in the honeycomb, the perforations in the sieve bottom must be fairly small. As for the size of the perforations, literature (Irandoust, S., Andersson, B., Bengtsson, E., SiverstrBoBm, M., Scaling Up a Monolithic Catalyst Reactor with Two-Phase Flow, Ind. Eng. Chem. Res., 1989, 28, page 1490) suggests a diameter within the range 0.5-1.5 mm. In the hydrogenation step of the hydrogen peroxide process, holes this small are easily clogged and are therefore a danger for the operation of the whole process. Moreover, an even distribution of the liquid is an apparent problem in this reactor, no matter what kind of distributor device is employed.